The invention relates to a process for the separation of gas mixtures which contain sulfur hexafluoride (SF6).
SF6-containing gases are used for widely varying purposes in industry. SF6/N2 mixtures or SF6/noble gas mixtures and SF6/air mixtures are used, for example, in the manufacture of windows as an insulating filler gas, and in the tyre industry as a pressure-resistant, noise-damping filler gas.
Mixtures of sulfur hexafluoride and nitrogen are used as an insulating filler gas for underground cables, see German Utility Model 297 20 507.2. Usually, these mixtures contain 5 to 30% by volume sulfur hexafluoride, remainder nitrogen to make up to 100% by volume.
Mixtures of sulfur hexafluoride and air and optionally CO2 are used as a protective gas cushion when casting magnesium. SF6 is usually contained in these mixtures in a quantity of 0.05 to 1% by volume.
SF6-containing exhaust air may also occur in industry.
It is desirable to work up these mixtures once they have been used, with the objective of re-using the SF6. The problem is that nitrogen or air takes up a large volume percentage in the gas mixture, and requires a large transport capacity. It is an object of the present invention to devise a process for separating the above gas mixtures which, upon re-use of the SF6 from the mixtures, requires only a small transport capacity.
A further object is to provide a suitable apparatus for performing the process according to the invention.
The process according to the invention for separating SF6-containing gas mixtures provides for the mixture to be contacted with hydrophobic zeolites having an SiO2/Al2O3 ratio (xe2x80x9cmodulusxe2x80x9d) of at least 80 and a pore diameter of 4 to 7 xc3x85 (0.4 to 0.7 nm), in order to adsorb SF6 preferentially. The substantially SF6-free nitrogen or the SF6-free air can be let off into the environment.
This simple way of performing the process is particularly suitable when relatively small quantities of gas mixture are to be separated or if the SF6 content is low, e.g. below 5% by volume. It is therefore particularly well suited for the mixtures of SF6 and air and optionally CO2 which originate from the casting of magnesium and usually contain 0.05 to 1% by volume SF6. If additional impurities such as SO2F2, SO2 etc. are contained therein, purification may be effected beforehand, such as washing with water or lye or sorption by means of e.g. Al2O3. This way of performing the process is also well suited for separating filler gases from insulating panes and car tires.
For example, the protective gas from the magnesium industry containing 0.2% by volume SF6, remainder air and CO2, is drawn off and the air is separated off. The sorbed SF6 and CO2 is recycled. With a few kilogrammes of adsorber material, virtually a whole week""s production of exhaust gas from a melting crucible can be purified. Magnesium can therefore be produced in a far more environmentally friendly manner than previously. The desorption time is not longer, or even shorter, than the sorption time.
Although the SF6 content is frequently 10% by volume for insulating-gas window panes, the extractor units frequently draw in secondary air, so that the effective SF6 content is lower upon sorption.
One embodiment of the invention relates to a process for the purification of SF6-containing gases, in particular air or waste air contaminated with SF6.
This aspect of the invention is based on the finding that SF6 can generally be separated out of gases using the selected zeolites.
Simple sorption tests show whether the separation occurs to the desired extent for a particular gas from which SF6 is to be removed. The separation effect is very successful if the molecules of the gas from which the SF6 is to be separated are larger or smaller than the pore diameter of the zeolite used. In this manner, for example, gases from production can be purified.
The method can also be used for the purification of air or exhaust air which is contaminated with SF6.
The improvement according to the invention resides in effecting the purification of air or exhaust air which is contaminated with SF6.
If additional impurities such as SO2F2, SO2etc. are contained therein, purification can be effected beforehand, such as washing with water or lye or sorption by means of, for example, Al2O3.
The purification of the exhaust air, for example from plants in which SF6 is produced or used is effected on environmental grounds.
The purification of air containing SF6 may be desirable because SF6 may have a disruptive effect on industrial processes. One field of application is therefore the purification of the air which is circulated in industrial plants.
The content of SF6 in such air is frequently very low (in the range of parts per million). For this reason, it is usually possible to dispense with the provision of membrane separation beforehand. Furthermore, in view of the small quantities of SF6, although it is possible to regenerate the laden sorbents, they may also be disposed of in the laden state.
To improve the purifying action, it is of course also possible to arrange a plurality of adsorption stages (2, 3 or more) in series.
This embodiment according to the invention is distinguished by a high purifying action for air contaminated with SF6. The purified exhaust air or air can be let off harmlessly into the environment or be passed harmlessly into plants in which SF6 would have been a disturbing contaminant and serve as ambient air therein.
The process may for example also be applied to air or exhaust air to which SF6 is added, e.g. as a tracer substance. Even air or exhaust air in plants manufacturing or using SF6 can be purified if this air or exhaust air is contaminated with SF6.
One preferred embodiment relates to the working-up of SF6/N2 mixtures, with reference to which the invention will be explained further.
If relatively large quantities are to be separated, or if the SF6 is relatively large, the SF6/N2 mixture may first be subjected to low-temperature treatment. Preferably the mixture is cooled to a temperature in the range from xe2x88x9270xc2x0C. to xe2x88x92110xc2x0 C., in particular to a temperature in the range from xe2x88x9270xc2x0C. to xe2x88x92100xc2x0 C. SF6 which only has a low content of N2 then condenses out. Furthermore, a gas phase remains which predominantly consists of nitrogen with low contents of SF6. The gas phase obtained in this manner is then separated by absorption as described above, so that nitrogen substantially free of SF6 is obtained and can be let off into the environment. The SF6 can be recycled.
An alternative, preferred embodiment for relatively large quantities of gas mixture, or those gas mixtures which have a higher content of SF6, will be described below. This embodiment provides for the combination of membrane separation processes and adsorption. It is highly suitable for mixtures of SF6 and N2, for example from underground cables, which have an SF6 content of 5 to 30% by volume.
This embodiment of the process according to the invention provides for an SF6/N2 mixture to be separated in at least one membrane separation stage into a retentate with an increased content of SF6 and a permeate with a reduced content of SF6, and for the permeate to be passed into at least one adsorption stage with the hydrophobic zeolites described above for further separation. It is preferred to provide two or more membrane separation stages and two or more adsorption stages.
The pressure on the entry side of the membrane or membranes is usually greater than the ambient pressure. For example, the gas mixture to be separated may be supplied at a pressure of up to 20 bar. If a plurality of membranes are provided, a compressor is arranged in front of each membrane. The permeate will then usually have a pressure which corresponds approximately to the ambient pressure upon entry into the adsorber stage. If desired, the permeate may be compressed before entry into the adsorber stage. This is however not essential. It is simplest to feed the permeate into the adsorber stage at the pressure which results from the membrane. The pressure is then usually up to 4 bar (absolute), preferably up to 2 bar (absolute).
If two membrane separation stages are provided, expediently the following guidance of the gas streams is provided: the mixture to be separatedxe2x80x94for example, a mixture of sulfur hexafluoride and nitrogen containing 20% by volume SF6 from underground cablesxe2x80x94is fed to the first membrane. Since the membrane preferentially allows nitrogen to pass, a permeate with a high nitrogen content and a low sulfur hexafluoride content is obtained. The permeate is introduced into the adsorber, or into the first adsorber; the gas mixture leaving the first adsorber is then introduced into a second, and then possibly into a third, etc., adsorber. Experiments have shown that after passing through just one membrane and two adsorbers filled with hydrophobic zeolites a nitrogen containing less than 10 ppm SF6 is obtained. The retentate of the first membrane is introduced into an additional membrane. The permeate resulting from this second membrane is introduced into the first membrane. The retentate from the second membrane is sulfur hexafluoride with small amounts of nitrogen. It can be stored temporarily after liquefaction with a compressor, be re-used immediately or be worked up with further enrichment of the sulfur hexafluoride.
The process may be performed very flexibly in terms of the number of membranes and adsorber stages. Depending on to what extent the depletion of SF6 is to be effected, one, two or even more adsorber stages are provided.
The number of the membranes and the arrangement of the membrane cartridges will depend on whether a gas with a high or a low SF6 content is to be treated. With a larger number of membranes, the separation effect is greater, and the SF6 content in the permeate which is to be treated by adsorption is lower than when using a small number of membranes. The adsorber may then either be designed to be smaller, or regeneration is necessary at longer intervals. The costs in terms of apparatus may however be higher (larger number of compressors).
It has been determined that even just one or two membrane separation stages and one or two adsorber stages are sufficient to obtain a highly enriched sulfur hexafluoride and a nitrogen gas with at most traces of sulfur hexafluoride.
Organic, asymmetrical membranes are preferred. As is known, there are rubber-elastic membranes (xe2x80x9crubbery membranesxe2x80x9d) which effect separation based on the solubility of the permeate. Other membranes effect separation based on the diffusibility of the permeate; these are non-rubber-elastic, but rather crystalline, membranes xe2x80x9cglassy membranesxe2x80x9d; these latter membranes are preferred. The membrane may be of conventional form. Membranes in the form of a bundle of hollow fibre membranes are highly suitable. The membrane material may be made, for example, of polysulfone, polyetherimide, polypropylene, cellulose acetate, polyimide, polyamide, polyaramid or ethyl cellulose, as described in U.S. Pat. No. 5,730,779. Other usable membranes are described in U.S. Pat. 4,838,904. For example, polyimides, polycarbonates, polyesters, polyester carbonates, polysulfones, polyethersulfones, polyamides, polyphenylene oxides and polyolefins are very highly suitable. Preferably the polymer material contains polyester, polycarbonates and polyester carbonates. Polycarbonates which have been derived from a biphenol in which at least 25% of the biphenol units in the polymer chain are tetrahalogenated, the halogen being chlorine or bromine, are outstandingly suitable. Particularly preferred membranes have a polymeric matrix which has two porous surfaces and a layer which permits the separation of the sulfur hexafluoride from the other gas constituents. Such membranes are described in U.S. Pat. No. 4,838,904 (EP-A-0 340 262). If additional impurities such as SO2F2, SO2 etc. are contained in the gas mixture, purification may take place beforehand, such as washing with water or lye or with adsorbers. Each membrane stage may consist of a plurality of membrane cartridges (arranged in parallel).
The pressure on the entry side of the membrane or membranes is usually higher than the ambient pressure. For example, the gas mixture to be separated may be supplied at a pressure of up to 13 bar. Preferably the entry pressure is from 10 to 12 bar. If a plurality of membranes are provided, a compressor may be arranged in front of each membrane. The temperature is advantageously from 10 to 40xc2x0 C.
If three membrane stages are used, the separation effect is even better. Preferably the three membranes are connected as follows: the SF6/N2 gas mixture is fed as feed stream to the first membrane stage. The retentate is fed as feed stream to a second membrane stage. The retentate of this second stage is highly enriched SF6, and can be re-used. The permeate of the first membrane stage is fed as feed stream to the third membrane stage. The permeate of this third stage is N2, virtually free of SF6, and is let off into the environment after passing through the adsorber or adsorbers. The permeate of the second membrane stage and the retentate of the third membrane stage are introduced into the feed stream to the first membrane stage.
It was established that, with zeolites which do not meet the selection criterion according to the invention in terms of modulus and pore size, only poorer adsorption of sulfur hexafluoride or none at all is possible, or that they are substantially less selective. A pore diameter of 5 to 6.5 xc3x85 (0.5 to 0.65 nm) is particularly advantageous.
It is possible to regenerate the adsorbents by lowering the pressure (pressure alternation adsorption) and optionally allowing heat to act on the coated adsorbents. The sulfur hexafluoride released may for example be fed to the feed stream into the membrane separation plant.
The process according to the invention is distinguished by optimum splitting of the SF6/N2 mixture or SF6/air mixtures. The purified nitrogen or the purified air may be let off harmlessly into the environment. The recovered sulfur hexafluoride may either be re-used immediately or after further purificationxe2x80x94optionally in liquefied form. The apparatus to be used in the [process] according to the invention may be in mobile form. In this case, the gas mixture, which originates for example from underground cables, may be separated on the spot.
The invention also covers an apparatus. This apparatus for separating sulfur hexafluoride/nitrogen or SF6/air mixtures comprises one, two, three or more membrane separation stages with membranes which are preferentially permeable to nitrogen or air, and one, two or more adsorbers with a bed of zeolites having a silicon dioxide/aluminium oxide ratio (modulus) of at least 80 and a pore diameter of 4 to 7 xc3x85 (0.4 to 0.7 nm). What has been stated above applies with regard to the number of membrane and adsorber stages. A compressor is arranged before each membrane stage. A preferred apparatus, as explained in FIG. 1, has two membrane separation stages and two adsorber stages. It further comprises a feed line for the gas mixture to be separated, which line is connected to the inlet into the first membrane separation stage, a connecting line between the first and second membrane separation stages which is intended for introducing the retentate from the first membrane separation stage into the second membrane separation stage, a connecting line between the second and first membrane separation stages, which serves for introducing the permeate of the second membrane separation stage into the first membrane separation stage, a line for removing the retentate from the second membrane separation stage, from which retentate with a high SF6 content can be removed, a line for feeding the permeate of the first membrane separation stage into the first adsorber, a line for feeding the gas leaving the first adsorber into the second adsorber and a line for removing the substantially SF6-free nitrogen gas (or SF6-free air) from the second adsorber.